Railroad ties are used to support the rails of a railroad. Ties are of a given length and placed at regular intervals for the entire length of the rail system. Switch ties are longer to provide a wider base where switches are installed to switch rail transportation to a different line.
Railroad ties are traditionally constructed of wood. Wooden ties, however, eventually decompose. Decomposition is more rapid in wooden ties in contact with moisture in a hot wet climate. Insects and bacteria consume the wood and weaken the structure. Ties also crack due to absorbed water freezing and thawing and are subject to damage by equipment. When a tie deteriorates to the point it no longer sufficiently supports the rail, transportation along the rail may be disrupted until the tie is replaced. Replacement of ties consumes resources including time and profits.
Ties used in railways in underground operations, such as in tunnels and mines, are exposed to harsh conditions, including standing water, increased humidity, heavy loads and acidic conditions. Such ties deteriorate at a rapid pace due to these conditions.
Alternatives to plain wooden ties have been proposed. Treated wood, such as pressure treating and the addition of chemicals, including chromated copper arsenate or creosote, increases the wood's resistance to insects and decay. The most common treated wood currently used in the railroad market is wood treated with creosote. While treating wooden ties increases the tie's resistant to insects and decay, handling and cutting of pressure treated lumber carries health risks, and the use of creosote-soaked wood products is now banned in several states.
The cost of wood used to make railroad ties has also increasing due to decreasing supplies. Other types of materials have been developed as substitutes for wooden ties. Cast concrete ties are in use, but expensive to buy and very labor intensive to make and install. Concrete also breaks down over time when subject to freezing temperatures and acidic conditions, and cannot be inserted into track with existing wood ties.
Steel ties have been developed and are commonly used in mine and tunnel railways. A steel tie connects to and secures the rails with tie plates. Steel ties, however, typically lack good support, which shortens their lifespan compared to that of wooden ties. To add support, steel ties can be bolted to a base, creating an “iron clad tie.” Iron clad ties are useful for increased weight and wear and tear on a line and currently include many shapes, typically called flat, trough, grooved, roof, and box ties.
Currently, the only bases available are wood and concrete ties, in that the top of the base must conform to the shape of the steel tie. The shaping of the wood is expensive, shaping of rested wood exposes workers to harmful chemicals, and treated wood decomposes and must eventually be replaced. Concrete also is expensive and eventually breaks down in the elements. As relevant to mine use, convention height wooden or concrete rails are impractical as iron clad ties in that the total height of a tunnel may not allow clearance of equipment transported on rails supported with conventional ties.
Composite railroad ties are a new and growing segment of the railway market. Composite ties are formed from polymeric blends that may include cellulose, chemicals, other resins and fillers that are heated and molded or extruded. Polymeric railroad ties will not rot, crack, warp, or splinter.
A typical polymer in composites is polyolefin. Polyolefin monomers are the lower olefins: ethylene, propylene, butylene and isoprene. Polyolefins are made by joining these monomers to form long-chain polymers, such as polypropylene and polyethylene. Polyolefins are thermoplastic polymers, in that they become elastic upon heating and firm when cool, and, upon reheating and re-cooling, do not becoming brittle.
Composite ties are denser than wood and maintenance free in that they are waterproof and unaffected by insects, bacteria and molds. Composite ties, however, are sometimes heavy and difficult to install.
A need exists for an easy to install, lightweight, weather resistant tie able to serve as a base for a steel tie, which is insect, bacteria, mold and chemical resistant, eliminates environmental concerns and reduces the exposure of workers to hazardous materials. A need exists for a tie capable of supporting a steel tie for use in railways in mines, tunnels and the like.